correct float height measuring procedure?

If it's broken or just needs tweaked

Moderators: Volker_P, tidd650

jonathanwhitwell
Posts: 4
Joined: Tue Dec 01, 2009 12:50 am

correct float height measuring procedure?

Postby jonathanwhitwell » Thu Dec 17, 2009 8:50 pm

Hello. I'm trying to fix up some carburetors I've got from a 1980 CB650C. They're the keihin kind with the mechanical slides. The clymer manual says the float height should be .5 inches (12.5 mm), but it's a bit ambiguous about how to measure that. I measured from where the fuel bowl seats (bottom of carb body) to the flat part of float closest to the front of the bike (if they were on the bike of course). The attached file is a picture of exactly how I measured it. When I took this measurement, the tang was just barely touching the float valve (needle valve). If someone knowledgeable could look at the picture and let me know if I'm measuring correctly, I'd appreciate it.
file:
100_0543.JPG
picture of how I measured float height
100_0543.JPG (75.68 KiB) Viewed 5882 times


Thanks in advance.
-Jon

User avatar
Volker_P
Posts: 5305
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: correct float height measuring procedure?

Postby Volker_P » Fri Dec 18, 2009 2:36 am

No need to worry. According to the below drawing in my German manual your measurement is correct.
BTW, interesting circular cover. Is there an air cut valve behind? I know this valve only from the manuals, never seen one installed. But I think also all my valve chamber covers look different from that.
Attachments
CB650_floater_level_piston_[PD5]_carbs.JPG
CB650_floater_level_piston_[PD5]_carbs.JPG (11.25 KiB) Viewed 5870 times
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

jonathanwhitwell
Posts: 4
Joined: Tue Dec 01, 2009 12:50 am

Re: correct float height measuring procedure?

Postby jonathanwhitwell » Fri Dec 18, 2009 2:45 am

Wonderful. The float heights were drastically wrong originally. It looks like the previous owner tried to get the bowls to stop leaking by setting the floats way too low. Maybe the bike will run this time around. I'll let you know when I try it again in a few days. Thank you.

Yes, there are working air cut-off valves behind the cover, and the passages for it are all functional (I checked with carb cleaner spray). I'm not sure how important they are though, there' nothing mentioned about them in my clymer manual.
-Jon

User avatar
Volker_P
Posts: 5305
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: correct float height measuring procedure?

Postby Volker_P » Fri Dec 18, 2009 3:37 am

Also have a close look at the jets, for size as well as for possible traces of manipulation. People playing around with very "exotic" floater levels are always suspicious as this hardly makes sense.
Air cut valves are sometimes required to avoid popping when releasing the throttle at speed. But seems no problem for CB650's as many CB650 carbs don't have them and also the passages are not drilled through. Possibly yours are early (79?) carbs. Which carb number?
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

jonathanwhitwell
Posts: 4
Joined: Tue Dec 01, 2009 12:50 am

Re: correct float height measuring procedure?

Postby jonathanwhitwell » Fri Dec 18, 2009 11:40 am

The carb number is PD50B. And I think you were right about the jet size! My book says the main jet is supposed to be a 90, but the main jet on the carburetors says 100. That seems really odd because the motorcycle has its original 4-4 stock exhaust pipes and stock air filter, air box, and cover. The pilot jet says 35 on it (if the pilot jet is the pressed in one directly in front of the needle/main jet assembly). My book doesn't say what the pilot jet size is supposed to be, so I don't know if 35 is correct or not. I also couldn't find any marking on the needle jet.

Also, I have another question for you: I got a set of carburetors from a 79 cb650 (part # PD50A). Among other things, I needed the #1 carb body from it. The bodies look identical except for the part number. Do you know if it's ok to put one of the PD50A bodies on my rack of PD50B bodies? The pilot jet sizes match, the fuel bowls are interchangeable, and both sets of carburetors have the air cut off valves w/ functional passages.

Thanks for the help!
-Jon

User avatar
Volker_P
Posts: 5305
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: correct float height measuring procedure?

Postby Volker_P » Tue Dec 22, 2009 3:27 am

Thanks for the info.
So possibly the P.O. tried to lower the floater level to compensate for too large mains. PD50A and PD51A mains are supposed to be 92's.
Idle jets are pressed in and hard to change, don't know the size however. Will try to have a look.
Sorry, all my carbs are PD51, never had one of the PD50's. I'd say you could try to exchange complete separated single carbs within different racks, but avoid changing sliders/pistons between different used carbs (no matter whether different number or not). Exchange of not moving parts should be no problem if they fit, but make sure whether the dimensions are really exactly the same. Before separating carbs, measure throttle cam and choke cam diameters to avoid surprises. Honda can be very annoying with things like that.
A fuel consumption within 50-60mpg can be taken as a hint that nothing is seriously wrong with the carbs, however one still should have a look on the plugs. A bit of white is all right, but too lean is dangerous for the engine. Check out with the look of the plugs between cylinders whether you can keep the 92 main in the PD50A carb or go also to 90 like for the PD50B's.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

User avatar
Volker_P
Posts: 5305
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: correct float height measuring procedure?

Postby Volker_P » Wed Dec 23, 2009 1:26 am

I can confirm the size of the idle jets of my PD51's is 35.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: correct float height measuring procedure?

Postby Rocket Man » Sun Aug 11, 2013 11:40 am

:roll: :shock:

I recently bought a 1980 CB650C in very good shape. Long story short, I have the carbs off for a second time due to a missing accelerator pump O-ring and very low operating temperature on number 2. It will not run much lower than 3000 rpm. I cleared number 4 accelerator pump orifice and now need to adjust the floats.

http://cosky0.tripod.com/imagelib/siteb ... et=tlx_new

According to the manual online, the measurement is from the float nearest the hinge pin.

If I were to do it the way this thread shows, it would be a very different level. Please advise...

User avatar
Volker_P
Posts: 5305
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: correct float height measuring procedure?

Postby Volker_P » Tue Aug 13, 2013 1:32 am

Welcome here! :D
In fact the proceeding shown in the picture and drawing above is the way you do it. With regard to accuracy, it does not make that much sense to measure close to the axis of rotation.
However the difference should be not that severe as the float is quite horizontal in this position.
Unless someone mounted it upside down ... (which is possible!) :wink:
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

User avatar
Bongskit
Posts: 160
Joined: Tue Mar 05, 2013 8:08 pm

Re: correct float height measuring procedure?

Postby Bongskit » Tue Aug 13, 2013 3:20 pm

Image

works for me.can be adjusted with carb on the bike.

Rocket Man wrote:If I were to do it the way this thread shows, it would be a very different level. Please advise...


I find using ruler or vernier is way off especially with a carb of unknown history, and you have to bring the carb down again if it overflows or you doubt your measurement.

Heres my actual float height using the method described above

Image

User avatar
Volker_P
Posts: 5305
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: correct float height measuring procedure?

Postby Volker_P » Wed Aug 14, 2013 1:58 am

Yes, that's the way to verify what you really have.
Some floaters are actually divers. :roll:
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues


Return to “Technical Discussion”

Who is online

Users browsing this forum: No registered users and 10 guests